Why does weight distribute evenly among both side of the body?



For example, why one leg or arm *usually* isn’t visible larger than the other

In: Biology

I’m not exactly sure what your asking, but I’ll try to answer anyway.

If you’re wondering how / why both sides of our bodies are similar weight and density, or why we are symmetrical, I can answer it with one easy word: evolution.

As humans, we run the fastest, the longest, and function at best when our weight is evenly distributed. We don’t have to have that one heavy leg that we need to drag, or have to awkwardly lean because one arm is huge compared to the other. We can efficiently function when everything is equal size and weight.

The reason why the majority of people have it is because our ancestors only mated with those who were strong, fast, and able. Symmetrical people were the strongest, fastest, and most able. I don’t mean that we started out unsymmetrical, but it was more like whenever someone was born unsymmetrical, they often didn’t reproduce to pass down any unsymmetrical traits.

Not sure if this answers, but hope it helps.

If it didn’t work that way, fat people would be lopsided and have trouble keeping their balance. So any sort of genes that would cause lopsidedness would very quickly get removed from the gene pool.

It doesn’t. Nobody has body weight evenly distributed, nor are their limbs the exact same length. Our hips are just well suited to rolling into whatever position they need to in order to balance the body properly.

You can see this clearly if you stand on one of those scales that measures fat, muscle, and bone in each limb. Each of your limbs will be different. For most people their dominant side will have more muscle.

As to the limb length. When it is really bad people wear one shoe with a lift. Arm length doesn’t really matter if it is different.

Both sides follow the same inherited building plan, because all cells contain the same DNA. They don’t have much of a choice, really.

In practice, it doesn’t always work out, because of, well… that’s life, I guess. Even babies aren’t 100% symmetrical. Not to mention the possibility of one side growing stronger because of right/left handedness, occupation, injury on the other side.

So we aren’t perfectly even on both sides. I even bet one of your shoes wears and tears quicker than the other, or at least according to a different pattern. Just go to a good shoe store, they’ll tell you which one.


Simpler example – look at a leaf. Cut it in half vertically. Both halves looks identical. Genetic copying of the same instructions is much more efficient.